Meebo, the Web-based instant-messaging application, now has an add-on for the Firefox Web browser. If you use Firefox, you can now IM all your friends from a side-pane, and drag links to anyone on your buddy list. When you get a new IM, you get a visual notification (such as a blinking tab or a pop-up toast).

Meebo is a completely Web-based IM service, unlike Skype or AIM or Yahoo Instant Messenger, which are most often accessed through separate clients. Meebo also bridges all the major IM services (AIM, MSN, Yahoo, Gtalk, ICQ, and Jabber). But up until now (unless you are one of the five people out there who use Flock), you had to go to Meebo.com to use it or go to a partner Website that embeds the Meebo widget.

Meebo already gets 6 million monthly logins (one million of those being daily), and 140 million messages a day are going back and forth. That number should keep rising with this add-on. Now users (at least those with Firefox) don’t have to go through the hassle of opening a new tab or another Web browser window to send an IM. Now your IM can be built into your browser, with a side pane that pops open when you get a message, and disappears when you don’t need it.